It’s a phrase I’ve heard many times while conducting site consultations on sustainable manure and pasture management. Yes, this is largely true; horses will avoid weeds in pastures where there is abundant grass forage available. Problems generally arise when pastures have been overgrazed and horses have no option but to seek out alternate food sources. It’s important for horse owners to know which plants to avoid and how to manage those already in our pastures.

Horses, as with most livestock, devote most of their time to eating and are designed to graze for the majority of the day. Horses do not have the intellect to stop eating when they have met their nutritional requirements and will continue to eat even if it causes digestive upset or laminitis. Horses will normally graze between five and ten hours a day, but if the quality of the pasture is low, they will often graze much longer.

Horses are also very selective grazers and have top and bottom teeth, unlike cattle, sheep, and goats which have bottom teeth only and graze in a very different fashion. Horses tend to pull grasses out of the ground and will graze down portions of the pasture while leaving other areas tall and overgrown. These bare spots allow areas for weeds to colonize, making it more likely that horses will be in contact with toxic plants.

What plants are toxic and how do they affect my horses?

There are a vast number of plants located throughout Canada that are toxic to horses in some respect. Many need to be eaten in large doses to cause much of an effect, while others require only a few mouthfuls. There are a variety of resources on plants toxic to livestock, but the Canadian Poisonous Plants Information System seems to be the most comprehensive. It lists over 250 poisonous plants found in Canada, their lethal dose (if known), and symptoms of poisoning.

Table 1 outlines ten of the plants in Canada most toxic to horses. A few of these, such as yew and oleander, are ornamentals that may be found in garden beds or around houses, but not in pastures. All of these plants contain a toxic substance that enters the bloodstream once ingested, causing a variety of symptoms that range from muscle spasms and convulsions to heart failure.

Water hemlock (Cicuta spp.) causes convulsions and can be fatal in just a few hours after only ingesting one root, while cocklebur (Xanthium strumarium) causes weakness, colic, and a weak pulse at a lethal dose of 0.75 percent body weight. Oak (Quercus spp.) leaves cause gastroenteritis and kidney disease if consumed in large quantities, but horses can develop a liking for the leaves, which poses a serious concern.

In addition to the plants listed in Table 1, there are many other toxic plants that vary in their mode of action and can cause a variety of sub-lethal effects including thiamine deficiency, nitrate poisoning, hydrogen cyanide poisoning, photosensitivity, and dermatitis. Some of these plants are outlined below.

Both field horsetail (Equisetum arvense) and bracken fern (Pteridium aquilinum), found throughout Canada in damp, acidic soils, contain the enzyme thiaminase, which destroys thiamine (vitamin B1) and causes a deficiency. Both plants are toxic when they are green in the pasture or when dry and incorporated into hay. Some of the symptoms of thiamine deficiency include weight loss, constipation, staggering gait, convulsions, and a raised pulse or heart rate.

Stork’s bill (Erodium cicutarium), spear-leaved goosefoot (Monolepis nuttalliana), barnyard grass (Echinochloa crusgalli), and witch grass (Panicum capillare) contain nitrates and can cause nitrate poisoning in horses if ingested in large doses. Nitrate poisoning is very rare, but the symptoms are severe: abdominal pain, diarrhea, shallow and rapid breathing, tachycardia, convulsions before death, and abortion in pregnant mares. Stork’s bill and spear-leaved goosefoot can be found across Canada, except in Newfoundland and PEI, and goosefoot has not been seen in New Brunswick. Witch grass is not found in Newfoundland either, but barnyard grass is located throughout Canada.

Johnsongrass (Sorghum halapense), arrow grass (Triglochin spp.), and cherry (Prunus spp.) can cause hydrogen cyanide poisoning, particularly in early spring. These plants contain cyanogenic glycosides that lead to salivation, trembling and convulsions, nervousness, rapid and difficult breathing, and an almond odour that can be smelled on the breath. Johnsongrass is commonly found in southwestern Ontario, while arrow grass and cherry can be found throughout Canada. Arrow grass is usually located in marshy, alkaline soils.

A number of other commonly-found plants have been known to cause photosensitization (sensitivity to light) and dermatitis (inflammation of the skin cells) in horses. St. John’s wort (Hypericum perforatum), alsike clover (Trifolium hybridum), buckwheat (Fagopyrum esculentum), and puncture vine (Tribulus terrestris) contain photodynamic agents that circulate to the skin and cause oxidative injury to skin cells when exposed to light, which is of particular concern for fair-coloured livestock. Buttercup (Ranunculus spp.), stinging nettle (Urtica dioica), burdock (Arctium minus), and Canada fleabane (Erigeron canadensis) contain physical and chemical irritants that cause inflammation in the mouth, the nostrils, and intestinal tract. All of these are found throughout Canada except puncture vine, which is only located in BC and Ontario.

Red maple (Acer rubrum) and tansy ragwort (Senecio jacobaea) are two other toxic plants worth mentioning here since they are prevalent across Eastern Canada and parts of BC. The leaves of red maple contain gallic acid, which can cause hemolytic anemia within one to five days of ingestion. Dry, wilted leaves are the most toxic and increase in toxicity throughout the summer. Ingestion of as little as 1.5 to three grams can cause symptoms such as depression and anemia. Tansy ragwort, a common pasture weed in eastern Canada as well as BC, contains pyrrolizidine alkaloids, which can cause irreversible liver cirrhosis and symptoms such as weakness, fever, staggering gait, and jaundice.

The symptoms and diseases caused by toxic plants sound terrifying, but it is important to remember that contact with toxic plants can be reduced and often eliminated by maintaining your pasture to prevent overgrazing.

How can I prevent these weeds from getting into my pasture?

“Nature abhors a vacuum” is an idiom that has been passed down since 485 BC. In terms of pastures, if a vacuum – a bare spot with no grass – is created in a pasture by overgrazing, it will be filled with something in order to abide by the laws of nature. Generally, natural processes will put weeds in the bare spots because they tend to colonize an area quickly. By creating a lush pasture with lots of grass coverage, it is possible to out-compete weeds and prevent their establishment.

The carrying capacity for most pastures is about one horse per acre. This can vary depending on the annual rainfall of the region, the soil structure in the area, and the amount of time horses are kept on pasture. The important thing to keep in mind is that pastures should never be grazed below three to four inches of grass height. Cutting grass off below this height compromises the root structure of the plant, resulting in grass that is far less likely to recover. It is also important to keep animals off wet pastures since their hooves can cause severe damage and create bare patches.

If you have many horses in a small area, it can be highly beneficial to rotate your horses around the pasture. You can divide pastures into two, four, six, or as many areas as is necessary to allow areas at least three weeks of rest from grazing to recover. Also, frequent mowing of pastures will prevent weed establishment and promote even pasture growth.

Good pasture management that includes techniques such as mowing and rotating horses into different areas will allow your pasture grass to thrive and discourage the growth of weeds and toxic plants. Photo credit: Mary R. Vogt

As long as horses have enough grass to eat they will be highly unlikely to eat potentially poisonous weeds in the pasture. By maintaining adequate grass growth and coverage you will be able to lower your feeding costs, ensure weeds are out-competed, and provide your animals with enough grass to deter them from eating dangerous weeds.

Where can I find more information?

The following websites contain additional information on poisonous plants in Canada:

This is by no means an exhaustive list of poisonous plants in Canada; it is highly recommended that you take the time to review some of the sources provided to see if your horse(s) may risk exposure to plants that could present harm. It is also important to always check with your veterinarian if you feel that your animal may be exhibiting symptoms of plant poisoning.

This article was originally published in the July 2011 issue of Canadian Horse Journal.

Main Photo - horse grazes in a field laden with buttercups, which are toxic to horses. While most horses will avoid toxic plants and weeds in a field with plenty of grass, horses on overgrazed or poor quality pastures may eat toxic plants and can become sick or die.